Using the ScriptManager of Master page in content page

This can be done using ScriptManager.GetCurrent() method.

Use this static method to determine whether a ScriptManager control is on a page, or to access the properties and methods of a ScriptManager control when you do not know its ID.

ScriptManager scriptManager;

if (ScriptManager.GetCurrent(Page) == null)
{
     scriptManager = new ScriptManager();
     scriptManager .ID = "ScriptManager1";
}
else
{
     scriptManager = ScriptManager.GetCurrent(Page);
}